Warming Sign? Larger dead zones form off Oregon coast

February 21, 2007 at 2:03 pm

Unprecedented recent changes in the yearly pattern of ocean currents off North America’s West Coast have wreaked havoc on aquatic ecosystems there. Those changes, which have triggered the formation of large areas of oxygen-poor water along the Oregon shore, may be a troubling symptom of Earth’s warming climate, a group of scientists says.

MUSSELED OUT. Poorly oxygenated waters off the coast of Oregon in the summers of 2005 and 2006 stifled the growth of sea life, such as the mussels and barnacles seen here. Lubchenco

Fisheries along the western coasts of North America, South America, and Africa account for less than 1 percent of the world’s ocean area but produce more than 20 percent of its wild-caught fish. In those areas, winds that blow toward the equator at certain times of the year bring cool, nutrient-rich waters to the coastal shallows, says Jane Lubchenco, a marine biologist at Oregon State University in Corvallis. In the past few years, however, productivity of some species has decreased precipitously, she and her colleagues reported last week in San Francisco at the annual meeting of the American Association for the Advancement of Science.

For example, large areas of oxygen-poor water, often called dead zones (SN: 6/5/04, p. 360: Dead Waters), now form each summer off Oregon. Such zones first appeared in 2002, but during the past 2 years, they were longer lasting than before and had lower oxygen concentrations, says Lubchenco.

John A. Barth, a physical oceanographer at Oregon State University, notes that in 2005, the annual onset of winds that blow toward the south off the Oregon coast was delayed from mid-April to mid-May. Lacking their usual input of nutrients, the plankton at the base of the region’s aquatic food chain didn’t thrive, and many plankton-eating creatures starved. When the winds finally kicked in, they brought an excess of nutrients that fed profuse, widespread blooms of algae that later died and decomposed, robbing the water of oxygen.

That double whammy of starvation and suffocation struck ecosystems hard, Barth reports. For instance, the number of young barnacles that he and other scientists found off the Oregon coast between June and August 2005 was 34 percent of normal. The tally of young mussels ended up at 17 percent of average in other years. Underwater cameras showed large numbers of dead Dungeness crabs in many areas.

In 2006, by contrast, winds blowing toward the south appeared earlier than normal and brought nutrient-rich waters to shore all summer long, fueling algal blooms that again devastated the ecosystem.

In the past 2 years, the breeding seasons of many seabirds along the West Coast have been disrupted, says Julia Parrish, a marine biologist at the University of Washington in Seattle. In the Farallon Islands off California, auklets abandoned their nests and produced no chicks, presumably for lack of food. “These birds were stressed out when they should have been pigging out,” she noted at the meeting.

Analyses of recent wind patterns suggest that year-to-year variability in the winds off Oregon is getting stronger, says Barth. The wind changes that the scientists have noted are consistent with what’s expected in response to climate change, but the specific cause of the change in wind patterns is unknown, Lubchenco says.